Copper makes up 4/7 of the mass of the alloy. How many kilograms of copper are contained in 280 kg of such an alloy.

To solve this problem, we introduce a conditional variable “X”, through which we denote the number of kilograms of copper contained in 280 kilograms of the alloy.

Then, based on the data of the problem, we will compose the following proportion: 280 kilograms of alloy are related to 1 as X kilograms of copper are related to 4/7.

Then, based on this proportion, we make the following equation: X x 1 = 280 x 4/7.

Solving this equation, we get X equal to 160 kilograms of copper.

Answer: 160 kilograms of copper is contained in an alloy weighing 280 kilograms.